Synthesis of trialkylbenzenes



2,920,117 v SYNTHESIS or TRIALKYLBENZENES Joe G. Hendrickson, Texas City, Tex., assignor to The American Oil Company, Texas City, Tex., a corporation of Texas No Drawing. Application November 20, 1956 Serial'No. 623,317

6 Claims. (Cl. 260--668) This invention relates to the synthesis of trialkylbenzenes and in particular it concerns the synthesis of symmetrical 1,3,5-trialkylbenzenes whose alkyl substituents contain from 1 to 3 carbon atoms per substituent.

The lower molecular weight symmetrical 1,3,5-trialkylbenzenes are highly desired for oxidation to aromatic polycarboxylic acids which are very useful in the production of alkyd type resins, plastics, and plasticizers. The symmetrical 1,3,5-trimethylbenzene (mesitylene) has an extremely high clear CPR-R octane number. Its blending octane number is among the highest of any pure hydrocarbon tested thus far, being 171. This high blending octane number contrasts with the lower blending octane numbers of other aromatics such as cumene (132), o-xylene (120), m-xylene (145), and p-xylene (146). Thusmesitylene, or fractions rich in mesitylene, are very valuable stocks for blending and producing high octane gasolines.

In accordance with the present alkylbenzenes whose alkyl substituents contain from 1 to 3 carbon atoms per substituent are synthesized by the steps of (1) condensing a dialkylbenzene, whose alkyl substituents contain from 1 to 3 carbon atoms per substituent, with an aldehyde having from 1 to 3 carbon atoms per molecule and thereby producing an alkyl substituted diarylalkane, (2) contacting the diarylalkane with an isomerization catalyst under conditions to. cause isomerization of the alkyl substituents around the benzene ring, and (3) cracking the isomerized diarylalkane to split it into two polyalkylbenzene molecules. One of the polyalkylbenzene molecules which is formed is a dialkylbenzene and the other polyalkylbenzene molecule is a 1,3,5-trialkylbenzene. The 1,3,5 trialkylbenzene has as its additional alkyl substituent an alkyl grouping having the same number of carbon atoms as were contained in the aldehyde employed in the condensation step (step 1). The dialkylbenzene molecule formed in the cracking step can be recycled to the condensation step and is ultimately converted to 1,3,5-trialkylbenzene. The invention is very suitable for preparing 1,3,5-trialkylbenzenes wherein each of the alkyl substituents attached to the benzene ring have the same number of carbon atoms, preferably no longer than two carbon atoms in length. By carrying out the cracking of the diarylalkane in the presence of hydrogen, usually using at least one mol of hydrogen per mol of diarylalkane and employing a hydrocracking catalyst, 1,3,5-trialkylbenzenes having saturated alkyl substituents are formed. If no hydrogen is employed in the cracking step, alkenyl polyalkylbenzenes can be produced. As an illustration of the invention, 1,3-dimethylbenzene (or a mixture of isomeric C aromatics) is condensed with formaldehyde to form the polyalkyl-substituted diarylrnethane, the diarylmethane is then isomerized to shift the methyl groups around the benzene ring, and the isomerized diarylmethane is hydrocracked to split it into a molecule of dimethylbenzene and a 4' molecule of 1,3,5-trimethylbenzene.

' As feed stocks to this invention may be employed any invention, 1,3,5 -tri- Y United States Patent benzyl single dialkylbenzene isomer or a mixture ofisomers. It is preferred to use the 1,3-dialkylbenzene isomer in preference to the other isomers, since the isomerization step proceeds more smoothly with less tar and disproportionation products formed. The alkyl substituents attached to the' benzene ring should have from 1 to 3 carbon atoms per substituent and preferably are 1 or 2 carbon atoms in 'length. To simplify product recovery, each of the alkyl substituents attached to the benzene ring of the feed dialkylbenzene preferably should have the same number of carbon atoms. The process of this invention is highly suitable for producing 1,3,5-trimethyl benzene from a feed stock of 1,3-dimethylbenzene or from a mixture of C aromatic isomers.

In forming the alkyl substituted diarylalkane, the feed dialkylbenzene is condensed with an aldehyde having from 1 to 3 carbon atoms per molecule. Since the alkyl grouping which is introduced into the feed dialkylbenzene, by virtue of the three step process of this invention, will have the same number of carbon atoms as were contained in the aldehyde employed in the condensation step, it is possible to prepare specific 1,3,5-trialkylbenzenes by choosing a suitable aldehyde for use in the condensation step. The reactants in the condensationstep are preferably employed in a ratio of about 2 moles of dialkylbenzene per mole of aldehyde. Molar ratios of between 1:1 and 10:1, preferably between 2:1 and 5:1 can be used. Any of the conventional techniques for carrying out the condensation reaction can be used. A wide variety of acid condensation catalysts such as aryl sulfonic acids e.g. benzene sulfonic acid, toluene sulfonic acid, low molecular weight alkane sulfonic acids, percent sulfuric acid, boron trifiuoride, zinc chloride or the like can be employed. Toluene sulfonic acid is preferred. When using 80 percent toluene sulfonic acid as the condensation catalyst, molar ratios of dialkylbenzene to aldehyde of 2:1 to 5:1 are satisfactory. Between 0.5

and 3 parts of the acid per part by weight of aldehyde may be used. With this catalyst a temperature of from 50 C. to C.', suitably the refluxing temperature, and a reaction time of from 0.5 to 10 hours e.g. 2 to 4 hours, is satisfactory.

In a typical operation the bulk of the hydrocarbon reaction products from the condensation step is separated from the bulk of the catalyst. Thereafter the hydrocarbons are preferably fractionated to recover unreacted dialkylbenzenes and to separate the diarylalkane from the minor amounts of higher molecular weight condensation products which are often produced in the condensation step. The diarylalkane, preferably diluted with an aromatic such as benzene as a precaution against solidification of the diarylalkane, is then preferably processed to remove any catalyst contained therein by techniques such as neutralization followed by water washing, filter- .ing through adsorbent solids such as Attapulgus clay, etc.

from the process of this lnvention, essentially no 1,3,5- trialkylbenzenes would be produced, and the product i would consist essentially of 1,2,4-trialkylbenzenes. The

method of carrying out the isomerization step is not critical and prior art isomerization processes such as employ Friedel-Crafts. metal halides, e.g. BF AlCl AlBr etc. or equivalent catalysts can be used. For instance the :diarylalkane can be isomerized' using from 0.5 to lO'percent AlCl based upon therdiarylalkane. Either HCl or an organic halide serving as a source of HCl such as chloride or the like can be used in an amount between about 0.1 to 10 percent to promote catalyst I hydrocarbons.

processed to remove any catalyst contained therein by activity. When using AlCl together with an or- "ganiechloride as the source of HCl, isomerization temperatures of 20 to 200 C., preferably below 50 C. may be employed. Isomerization reaction times of from not from the cracking step contains a higher proportion of 1,3,5-trialkylbenzene'when the 'isomerization has been "carried out at a lower rather than at a higher temperature. After the isomerization step has been completed, the bulk of the catalyst is separated from the isomerized The hydrocarbons are then preferably techniques such as neutralization followed by water washing, filtering through an adsorbent solid such as Attapulgus clay, etc. before introducingthe isomeriz d diarylalkane into the cracking step.

The cracking step is carried out under conditions which cause cleavage of the carbon-to-carbon bond linking the two benzene rings. The cracking step can be carried out in the presence or absence of hydrogen. If it is carried out in the absence of hydrogen, the 1,3,5-trialkylbenzene produced from the initial dialkylbenzene will contain as its additional substituent, an unsaturated grouping if the aldehyde used in the condensation step contained 2 or 3 carbon atoms. For example, if acetaldehyde were condensed with 1,3-dirnethylbenzene, the di-(m-xylyl) ethane was isomerized, and the isomerized product was cracked in the absence of hydrogen, then each molecule of the diarylethane would produce a molecule of dimethylbenzene and a molecule of 1,3-dimethyl S-ethenylbenzene. If the cracking were carried out in the presence of hydrogen, saturation of the rupturing carbon-to-carbon linkage would occur and thus 1,3-dimethyl S-ethylbenzene would be formed.

When carrying out the cracking step in the absence of hydrogen, a cracking catalyst such as silica-alumina, sili ca-alumina-zirconia, silica-magnesia or the like can be used. These same catalysts can be employed when the cracking step is carried out in the presence of hydrogen, but it is preferred to use a hydrocracking or hydrogenation catalyst such as molybdena-alumina, cobalt molybdate-alumina, platinum-alumina, titania either unsupported or supported on alumina, nickel oxide or nickel sulfide either unsupported or supported, nickel tungsten sulfide or the like. Itis preferred to carry out the cracking step in the presence of hydrogen, i.e. hydrocracking, while using a hydrocracking catalyst of the type described since the reaction is cleaner, less coke is formed, etc. Suitable cracking conditions comprising a temperature of 350 to 650 C., between about 400 and 550 C. being very satisfactory, and a space velocity of about 0.5 to about 10 liquid volumes of diarylalkane per volume of catalyst per hour may be used. Pressures of atmospheric to 1500 p.s.i.g. can be employed. When a hydrocracking operation is used, at least about one mol of hydrogen per mol of diarylalkane is introduced into the cracking zone. amount of hydrogen, suitably about 3 to 10 mols of hydrogen per mol of diarylalkane. Also when the hydrocracking technique is used, it is preferred to employ .pressures of from 100 to 500 p.s.i.g. As an illustration,

the isomerized di-(mxylyl) methane, which is preferably diluted with benzene, is contacted with a molybdena-alumina catalyst (containing about 9 weight percent M at a temperature of about 525 C. and a pressure of about 100 p.s.i.g. while employing 4 moles of hydrogen per mol of the diarylmethane and 1.0 LHSV. The products from the hydrocracking step are recovered and the 1,3.5-trimethylbenzene is separated by fractionation or other suitable techniques.

Example I In this example m-xylene Was employed as feed and converted ultimately by the processof this invention to It is preferred to use more than this minimum mesitylene. The m-xylene was condensed with paraformaldehyde 2:1 molar ratio of aromatic to aldehyde) by refluxing in the presence of commercial anhydrous toluene sulfonic acid (amount of acid equal in weight to the amount of aldehyde) for about 4 hours. After dilution with benzene, the hydrocarbon layer containing the diarylmethane was decanted from the acid layer. The decanted'hydrocarbons were then passed through a layer of Attapulgus clay and a layer of silica gel. Thereafter the benzene solution of di-(m-xylyl) methane was thoroughly agitated with 1 weight percent AlCl and 1 weight percent benzyl chloride (amounts based upon diarylmethane) at a-temperature of about 30 C. for two hours. The hydrocarbon isomerization products were then separated from the catalyst and passed through a layer of Attapulgus clay and a layer of silica gel. Thereafter the isomerized hydrocarbon product was hydrocracked over a molybdena-alumina catalyst (9 percent M00 at a temperature of 525 C. and at 'p.s.i.g. while employing a hydrogen to diarylmethane molar ratio of 6:1 and a space velocity of about 0.4 liquid volumes of hydrocarbon/hour/volume of catalyst. An infrared analysis of the products from hydrocracking was made. It was found that approximately 70 percent of the trimethylbenzene fraction consisted of 1,3,5-trimethylbenzene, the remainder consisting essentially of 1,2,4-trimethylbenzene.

Example 2 The experiment set forth in Example 1 was repeated in essentially the same manner except that the initial dialkylbenzene employed as feed consisted of a mixture of the isomeric xylenes rather than pure m-xylene as had been employed in Example 1. An infrared analysis of the products from the hydrocracking step was made. It was found that approximately 53 percent of the trimethylbenzenes consisted of 1,3,5-trimethylbenzene, the remainder being essentially 1,2,4-trimethylbenzene.

Example 3 This example shows the effect of the isomerization step on the type of products which are produced upon hydrocracking. In this experiment, the procedural techniques of Example 1 were repeated in essentially the same manner except that the di-(m-xylyl) methane was not isomerized before it was hydrocracked. Upon infrared analysis of the products from trialkylbenzene, it was found that somewhat less than 9 percent of the trialkylbenzenes was 1,3,5-trimethylbenzene whereas 87- percent of the trialkylbenzenes was 1,2,4-trimethylbenzene.

It is evident from a comparison of the results obtained in Examples 1 and 3 that the isomerization of the diarylalkane is critically essential in producing the symmetrical 1,3,5-trimethylbenzene.

Thus having described the invention what is claimed 1s:

1. A process for synthesizing 1,3,5-trialkylberizenes whose alkyl substituents contain from 1 to 3 carbon atoms per substituent which process comprises condensing a dialkylbenzene whose alkyl substituents contain from 1 to 3 carbon atoms per substituent with an aldehyde contaming from 1 to 3 carbon atoms per molecule and formmg an alkyl substituted diarylalkane, contacting the diarylalkane with an isomerization catalyst and effecting lsomerization of the alkyl substituents around the benzene ring, subjecting the isomerized diarylalkane to crackmg conditions whereby the diarylalkane is split into two polyalkylbenzene molecules, one of which molecules is a dialkylbenzene and the other polyalkylbenzene molecule being a 1,3,5-trialkylbenzene whose additional "substituent has the same number of carbon atoms as were contained in the aldehyde employed in the condensation step.

2. The process of claim 1 wherein each of the alkyl substituents attached to the benzene nucleus of the feed dialkylbenzene have the same number of carbon atoms per substituent and wherein the aldehyde has the same number of carbon atoms per molecule as are contained in said alkyl substituents.

' 3. The process of claim 1 wherein the cracking step is carried out under hydrocracking conditions comprising the use of a hydrocracking catalyst and at least one mol of hydrogen per mol of diarylalkane.

4. A process for synthesizing 1,3,5-trimethylbenzene which comprises condensing a dimethylbenzene with formaldehyde and forming a methyl substituted diarylmethane, contacting the diarylmethane with an isomerization catalyst and effecting isomerization of the methyl substituents around the benzene ring, hydrocracking the isomerized diarylmethane in the presence of a hydrocracking catalyst and at least one mol of hydrogen per mol of diarylmethane whereby the diarylmethane is split into two polymethylbenzene molecules, one of which molecules is a dimethylbenzene and the other polymethylbenzene molecule being 1,3,5-trimethylbenzene.

5. The process of claim 4 wherein the feed dimethylbenzene is 1,3-dimethylbenzene.

6. The process of claim 4 wherein the feed dimethylbenzene is a mixture of C aromatic isomers.

References Cited in the file of this patent UNITED STATES PATENTS 2,403,757 Reeves July 9, 1946 2,422,171 Saunders Jan. 10, 1947 2,519,719 Sturrock et al Aug. 22, 1950 2,761,885 De Jong et a1. Sept. 4, 1956 2,819,322 Fetterly Ian. 7, 1958 2,836,632 Fetterly May 27, 1958 OTHER REFERENCES Welch et a1.: Journal Amer. Chem. Soc., vol. 73, pp. 4391-3, (1951). 

1. A PROCESS FOR SYNTHESIZING 1,3,K-TRIALKYLBENZENES WHOSE ALKYL SUBSTITUENTS CONTAIN FROM 1 TO 3 CARBON ATOMS PER SUBSTITUENT WHICH PROCESS COMPARISES CONDENSING A DIALKYLBENZENE WHOSE ALKYL SUBSTITUENTS CONTAIN FROM 1 TO 3 CARBON ATOMS PER SUBSTITUENTS WITH AN ALDEHYDE CONTAINING FROM 1 TO 3 CARBON ATOMS PER MOLECULE AND FORMING AN ALKYL SUBSTITUTED DIARYLALKANE, CONTACTING THE DIARYLALKANE WITH AN ISOMERIZSTION CATALYST AND EFFECTING ISOMERIZATION OF THE ALKYL SUBSTITUENTS AROUND THE BENZENE RING, SUBJECTING THE ISOMERIZED DIARYLALKANE TO CRACKING CONDITIONS WHEREBY THE DIARYLALKANE IS SPLIT INTO TWO POLYALKYLBENZENE MOLECULES, ONE OF WHICH MOLECULES IS A DIALKYLBENZENE AND THE OTHER POLYALKYLBENZENE MOLECULE BEING A 1,3,5-TRIALKYLBENZENE WHOSE ADDITIONAL SUBSTITUTENT HAS THE SAME NUMBER OF CARBON ATOMS AS WHERE CONTAINED IN THE ALDEHYDE EMPLOYED IN THE CONDENSATION STEP. 